The invention relates to a recuperator in a thermal power installation such as, for example, an air storage power installation, air from a storage cavern being heated by means of hot exhaust gases in the recuperator, which air is supplied to turbines after it has been heated.
Recuperators of this type are used for heating air which is intended for driving turbines, the air being heated by heat transfer from the exhaust gases of the turbines. They are employed in thermal power installations such as, for example, air storage power installations.
In air storage installations, the air from a storage cavern is heated. Power installations of this type are, for example, known from the Patent Specification WO 96/01942.
An important function of air storage power installations in the present-day electricity market is their use during peak load periods in which other power installations"" electricity demand is not sufficiently covered. Such peak load periods often occur at short notice and, for this reason, it is also necessary for air storage power installations to have the capability of being started up rapidly. This means that they can be started up again to full load from standstill or reduced operation as rapidly as possible.
A typical air storage power installation such as is in operation in Alabama, USA, for example, is shown in FIG. 1. The power installation 1 comprises a storage cavern 2, in which air is stored at a certain pressure and a typical temperature of approximately 35xc2x0 C. and, in addition, a recuperator 3, a generator G, two turbines with a high-pressure turbine 4 and a low-pressure turbine 5, upstream of which is respectively connected a high-pressure combustion chamber 6 and a low-pressure combustion chamber 7. When the power installation is put into operation, the stored air is supplied via the duct 8 to the recuperator 3. There, it flows through one or a plurality of sectors, each consisting of a pipework system, the stored air being heated to above 300xc2x0 C. by heat exchange with hot exhaust gases from the low-pressure gas turbine. After it has been heated, the air is supplied via a duct to the turbine group. The hot exhaust gases from the low-pressure turbine 5 are supplied via the duct 9 to the recuperator 3.
Recuperators of the prior art can also comprise a so-called selective catalytic reduction system (SCR) for the purpose of reducing pollutant exhaust gases such as, for example, NOx.
When starting up an air storage power installation, in particular in the case of a cold start after a fairly long outage period, the combustion chambers, the turbines and also the recuperator are, inter alia, started up, temperatures of between 300xc2x0 C. and 550xc2x0 C. being attained. During a cold start, it is then necessary to take account of the temperature distribution in the items of equipment in order to ensure risk-free operation. During the outage period of the installation, namely, the temperature distribution in the power installation components mentioned evens out and there is a reduction in their average temperature. During the subsequent start, the thermal load may only be applied gradually to the components because, otherwise, the component details such as the tubes of the recuperator, for example, are subjected to marked transient stresses.
In the recuperator of the prior art, as shown in FIG. 1, the heating of the air (relaxation or temperature response) takes place with delay. This is a consequence of the thermal capacity of the recuperator and is strongly marked, particularly in the case of high pressures such as occur in air storage power installations and in the case of high efficiencies. This effect makes the cold start of the high-pressure turbine more difficult and, under certain circumstances, imposes the need for the high-pressure combustion chamber 6 of FIG. 1. In addition, the delayed temperature response leads to a likewise delayed effective appearance of the selective catalytic reduction (SCR). The limitation to a gradual increase in the thermal load leads to a loss of time when starting up the air storage power installation, which makes it impossible for the installation function of covering peak times to occur at short notice and endangers the economic success of the air storage power installation per se.
Based on this prior art, the object of the present invention is to create a recuperator for a thermal power installation, which latter can be started up, without loss of time, to full load after an outage period or reduced power operation. In particular, the power installation is to be able to be put into operationxe2x80x94as compared with the prior artxe2x80x94both in a reduced start-up time and without risk, its start-up time being matched as far as possible to that of the turbines or, preferably, markedly bettering this start-up time.
This object is achieved by means of a recuperator according to claim 1 and a method for its operation according to claim 5. Special and preferred embodiments of the invention are given in the sub-claims.
A recuperator for an air storage power installation having at least one turbine such as, for example, a high-pressure turbine and low-pressure turbine, at least one combustion chamber and a generator, has a sector or a plurality of sectors in a recuperator casing, which sectors each exhibit a duct system for the heating of air by heat transfer from hot exhaust gases from a gas turbine, and which recuperator is connected to air supply ducts and transfer ducts to the at least one turbine. According to the invention, the recuperator has one or a plurality of external heat storage devices, which are connected between the sectors of the recuperator. In the case of a single sector in the recuperator, an external heat storage device is connected before and/or after this sector.
During a normal on-load operation, the external heat storage devices have the function of storing heat which is used during an installation outage to keep the individual sectors of the recuperator, together with the combustion chambers and turbines, sufficiently warm so that they do not cool. During the outage period of the installation, the heat storage devices ensure the heat which is necessary for maintaining a temperature distribution in the recuperator and the turbines, which temperature gradients are not larger than the critical temperature gradients which cause critical, transient stresses in the components of the recuperator and the turbines.
The external heat storage devices are expediently arranged outside the recuperator casing.
In a special embodiment of the invention, the one or the plurality of external heat storage devices are provided with water as the thermal heat storage medium or, in the case of higher temperatures, with a solid such as, for example, sand or stone.
In a method of operating the recuperator according to the invention, the external heat storage devices are, during the normal on-load operation of the power installation, brought to the temperature of the recuperator. During an installation outage, the recuperator, the turbines and the combustion chambers are then continuously or discontinuously flooded during a heat retention phase, by which means a temperature distribution is attained and maintained in the recuperator, the temperatures of the various sectors of the recuperator not falling below the temperature of the nearest heat storage device. This ensures that temperature gradients in the recuperator and the turbines remain smaller than a specified critical value.
In a special embodiment of the method according to the invention, the temperature distribution in the recuperator is held in such a way that the difference between the temperatures of the storage media of two adjacently connected heat storage devices remain smaller than the critical temperature with respect to transient stresses.
In one embodiment of the invention, the recuperator is flooded by air or air flows through it.
In one variant of the invention, the recuperator is flooded by air from the atmosphere, this air being supplied to the recuperator by means of a fan.
In a further variant, in which the thermal power installation is an air storage power installation, the recuperator is flooded by air from a storage cavern.
In a further special embodiment of the operating method, a flushing operation of the recuperator, the combustion chambers and the turbines takes place with preheated air after the flooding of the recuperator for the purpose of low-stress operation, in which the inner volume of the recuperator and the turbine group is exchanged several times. In this way, not only the components of the recuperator but also those of the turbines are protected from transient stresses. The power installation is subsequently started up to full load.
The recuperator according to the invention and the operating method according to the invention provide the advantage that the power installation can be started up rapidly to full load, because the temperature difference between successive heat storage devices lies within the risk-free region with respect to transient stresses for the materials of the components of the recuperator. By avoiding temperature differences of more than 160 K, transient stresses in the tube walls of the sector are avoided and the risks with respect to life of the associated tube walls of the ducts are avoided.
In particular, the power installation can be started up in a time which lie [sic] markedly below the start-up times of a normal gas turbine.
In addition, they provide the advantage that the high-pressure combustion chamber, which is arranged upstream of the high-pressure turbine in the power installations of the prior art, can be omitted without replacement. This provides the further advantage that the power installation produces less NOx which, in installations of the prior art, originate [sic] mainly from this high-pressure combustion chamber. The invention also provides the advantage that the SCR functions immediately.
In addition, no segmentation of high-pressure turbine rotor and stator is necessary.
The avoidance of large temperature gradients during start-up of the installation provides, finally, the avoidance of flaking of oxide layers on the surfaces of the recuperator components.
The recuperator and the method of operating it according to the invention can be applied in all types of thermal power installations such as, for example, gas turbine and air storage power installations. The invention is explained in more detail below using, as an example, an air storage power installation and the drawings.